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I'm not sure of my tier but I win most of my games (as the carry) with Sion at lvl 30 vs other 30s.
The thing that helps me succeed is getting kills early (with ignite) then rushing mejai's, sheen, and occasionally catalyst. Sometimes I won't even get boots until level 18 because I get so fed.
Focus on that for a few games and if you don't think its going to work you can try a tankier ap build. RoA first then Banshee's, Rylai's and Lich Bane in any order. Or you could get a couple RoA and then go from there.
As far as other heroes, I run my main, Kassadin, of the same rune book as Sion - cooldowns, mana regen, and magic penetration. However, I'm not sure you'll like Kassadin any better because he's harder to utilize from game to game than Sion imo.